Recombinant vectors have been constructed which combine elements of the prokaryotic plasmid PBR322 and the mammalian virus SV40. Coding sequences of prokaryotic or eukaryotic origin may be inserted into these vectors and the resulting secondary recombinants propagated in E. coli. After transfection of mammalian cells with the secondary recombinants, the inserted coding sequences are expressed constitutively under control of the SV40 early cistron. Coding sequences inserted to date include rabbit beta-globin and E. coli plasmid dihydrofolate reductase.